Of the electrons that are made available, not all of them will actually make it to the metal contact and generate electricity. This also cuts the efficiency because that heat energy is not being used for any useful task. Any energy above the band-gap energy will be transformed into heat. Įven from the light that can be absorbed, there is still a problem. Efficiency of a PV cell is largely effected by the amount of incoming light that can cause current to flow. The solar energy received by the Earth, and the corresponding energies of its photons. This means that ~1/4 of the light from the Sun is not sufficient enough to create electricity. From Figure 2 below it can be seen that the visible light and ultraviolet light have sufficient energy, but a good portion of infrared light cannot be used. The longest wavelength (which corresponds to the lowest energy) that is capable of removing an electron is 1.1 µ m. For silicon, the band gap energy is 1.12 electron volts. If it has sufficient energy it has a chance of "knocking" an electron loose and producing an electrical current. If a photon strikes a solar cell and has less energy than the band gap, it gets absorbed as thermal energy. Sunlight contains a wide spectrum of photons with different wavelengths and energy. For solar cells, this energy is provided by particles of light called photons, which are tiny packets of electromagnetic radiation released from the Sun.
The physics of semiconductors requires a minimum amount of energy to remove an electron from a crystal structure, known as the band gap energy. Unlike the carnot efficiency which limits the thermal efficiency of heat engines, the efficiency of solar cells is limited by something called the "band gap energy". Although some experimental solar cells have achieved efficiencies of close to 50%, most commercial cells are below 30%. For solar photovoltaic (PV) cells, this means the ratio of useful electrical energy they produce to the amount of solar energy incident on the cell under standardized testing conditions. Įfficiency is the comparison of energy output to energy input of a given system.
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